The present invention relates to a method, a computer program and an apparatus for determining a stereo signal.
A stereo microphone usually uses two directional microphone elements to directly record a signal suitable for stereo playback. A directional microphone is a microphone that picks up sound from a certain direction, or a number of directions, depending on the model involved, e.g., cardioid or figure eight microphones. Directional microphones are expensive and difficult to build into small devices. Thus, usually omni-directional microphone elements are used in mobile devices. An omni-directional or non-directional microphone's response is generally considered to be a perfect sphere in three dimensions. However, a stereo signal yielded by omni-directional microphones has only little left-right signal separation. Indeed, due to the small distance of only few centimeters between the two omni-directional microphones, the stereo image width is rather limited as the energy and delay differences between the channels are small. The energy and delay differences are known as spatial cues and they directly affect the spatial perception as explained in J. Blauert, “Spatial Hearing: The Psychoacoustics of Human Sound Localization”, MIT Press, Cambridge, USA, 1997. Thus, techniques have been proposed to convert omni-directional microphone signals to stereo signals with more separation as shown by C. Faller, “Conversion of two closely spaced omnidirectional microphone signals to an xy stereo signal,” in Preprint 129th Convention AES, 2010.
The weakness of the previously described method is that the differential signals have low signal-to-noise ratio at low frequencies and spectral defects at higher frequencies. The technique proposed in C. Faller, “Conversion of two closely spaced omnidirectional microphone signals to an xy stereo signal,” in Preprint 129th Convention AES, 2010, attempts to avoid these issues by using the differential signals (x1 and x2) only for computing a gain filter, which is then applied to the original microphone signals (m1 and m2), and which achieves a good signal to noise ratio (SNR) and reduced spectral defects.
This technique, however, is limited to a specific stereo image or a specific sound recording scenario.